I’m one of the developers on Mercury Reader. Here is a little bit of info about our tech stack – feel free to reach out if you have any questions!
Mercury Reader is a Chrome extension that runs purely client-side. We use:
• React – This is our favorite javascript library for building user interfaces.
• Mercury Parser – Extracting content from chaos. The Mercury Parser module powers the Mercury Parser API and is bundled to run client-side in Mercury Reader.
• Basscss – a collection of immutable utilities that helped us prototype at light speed. Warning, could change the way you write CSS.
• Aphrodite – Yep, inline styles. We use Aphrodite to add conditional styles based on users’ selected font size, typography, and theme. (I’m partial to large/sans/dark).
• Babel– Write futuristic javascript, run it in present-day browsers.
• Webpack – Once you get the hang of webpack, you’ll forget all about your old build tool.

So when Readability shut down in September, it seemed like clean and clear web reading could only be achieved with Safari's Reader View or inside of Pocket. But fortunately the folks at Postlight showed up and took over the Chrome Extension and relaunched it using their underlying Mercury technology — and relaunched Readability as Mercury Reader.
No Chrome browser is complete without this!
Update: it was pointed out to me that I failed to mention Instapaper. My bad!